Voltage-controlled transistors are known and used in many ways. They can be embodied for example as a FET, in particular MOSFET, or IGBT. Voltage-controlled transistors have a control input, which influences the switching path of the transistor depending on its voltage with respect to a reference potential. By way of example, a negative charging of the control input (gate in the case of a MOSFET) can prevent the movements of conduction electrons through the switching path of the transistor (between drain and source) and thus turn the transistor off or, if said transistor is regarded as a switch, open it.
Voltage-controlled transistors are often used for switching relatively large powers. However, the switching instants are usually determined by means of signal processing electronics with only a low power conversion. The control inputs of voltage-controlled transistors cannot be switched arbitrarily rapidly by correspondingly weak signals on account of their input capacitance.
Therefore, a (driver) circuit that is triggered by the weak signals is usually used for switching the voltage-controlled power transistors. For this purpose, such circuits have their own power supply, which they use to generate correspondingly strong switching signals.
A simple circuit for switching a voltage-controlled transistor with an operational amplifier is presented by way of example here. It comprises an operational amplifier having an open collector output, at which one of two states can be present. In one state, the output of the operational amplifier is connected to a reference-ground potential at low impedance; in the other state, the output of the operational amplifier is isolated from said potential at high impedance. These states can be referred to as logic zero and logic one. A DC voltage supply can be used for switching a voltage-controlled transistor. In this case, a first resistor, a so-called “pull-up resistor”, is connected between a supply potential of a DC voltage supply and the output of the operational amplifier. The output of the operational amplifier is connected to the control input of the voltage-controlled transistor. If the output of the operational amplifier is at low impedance (logic zero), then the control input of the voltage-controlled transistor is subjected to charge reversal to the reference-ground potential via the operational amplifier. If the output of the operational amplifier is at high impedance (logic one), then the control input of the voltage-controlled transistor is connected to the supply potential of the DC voltage supply via the “pull-up resistor” and can be subjected to charge reversal via said supply. Depending on the polarity of the DC voltage supply and the construction of the voltage-controlled transistor, this can correspond to the switching path of said transistor being switched on or off.